In exhaust gas discharged from an internal combustion engine such as a diesel engine, nitrogen oxides (hereinafter referred to as “NOx”) that may affect the environment are included. In known art, as an exhaust gas purification apparatus used for purifying the NOx, a SCR (Selective Catalytic Reduction) system is known. The SCR system adds a reducing agent, such as urea aqueous solution, into the exhaust gas, and selectively reduces and purifies the NOx in a reducing catalyst.
The urea aqueous solution used in the SCR system as a reducing agent has a characteristic in which the solution crystallizes after the solvent is evaporated, when the solution is placed at a predetermined temperature range or left for a long time. Further, there is also a possibility that the urea aqueous solution becomes frozen when it is cold. When the reducing agent solidifies inside reducing agent supply paths, a supply of the reducing agent into an exhaust gas passageway becomes insufficient, and reduction and purification of the NOx in the reducing catalyst do not progress sufficiently. Thus, there is a concern that unpurified NOx may be discharged into the air.
Then, an exhaust gas purification apparatus that has made the reducing agent supply paths less likely to be clogged has been proposed. More specifically, as shown in FIG. 8, the exhaust gas purification apparatus that has been disclosed includes pressure lowering means (a pressure regulator 338) for lowering pressure inside an upper space of a reducing agent container (a storage tank) 324 at least to atmospheric pressure, when it is determined, by operation determination means for determining an engine operating condition, that an engine is stopped. When the engine is stopped, the exhaust gas purification apparatus is structured such that the reducing agent existing in the reducing agent supply paths is reliably collected into the reducing agent container 324 (Patent Document 1).    Patent Document 1: JP-A-2006-132384 (claim 4, Paragraph [0021], FIG. 2)